The present invention relates to a hydrodynamic gas bearing, provided for the main shaft of a polygon mirror scanner or the like, for floating a sleeve relative to a shaft to constitute a radial bearing and a thrust bearing by pressure generated in dynamic pressure generating grooves provided in the radial bearing, and more particularly to a hydrodynamic gas bearing which generates no whirls or self-excited vibrations and provides stable rotation.
FIGS. 7 and 8 show an example of a conventional hydrodynamic gas bearing applied to a polygon mirror scanner.
Referring to FIG. 7 which is a sectional view showing the conventional hydrodynamic gas bearing, the lower end of a shaft 12 is fixed to the center portion of a main body 11 of the bearing. There are provided in the periphery of the shaft 12 a pair of dynamic pressure generating herringbone-configured hydrodynamic pressure generating grooves 12A. A peripheral groove 12B is provided in the region, between the herringbone-configured hydrodynamic pressure generating grooves 12A, in which a high pressure is generated. There are provided in the shaft 12 a communicating hole 12C and a throttling hole 12D for feeding pressurized gas under pressure from the peripheral groove 12B to the end surface of the free end of the shaft 12. There is provided a rotatable lid 17 positioned on the free end of the shaft 12. A flange 13B for mounting a polygon mirror 19 to a sleeve 13 is provided in the vicinity of the upper end of the sleeve 13. The lid 17 serves as a means for fixing the polygon mirror 19 to the main body 11 by a bolt. A rotor magnet 15 is mounted on the sleeve 13 and a stator 14 is fixed to the main body 11. The stator 14 and the rotor magnet 15 constitute a motor 16. A protective dust cover 20 having a glass 20A is fixed to the main body 11.
The operation of the hydrodynamic gas bearing of the above-described construction is described below with reference to FIGS. 7 and 8. Referring to FIG. 7, upon energizing of the stator 14 of the motor 16, the rotor magnet 15 is rotated. While the lid 17 and the polygon mirror 19 rotate together with the sleeve 13 at a speed as high as, for example, 30,000 r.p.m., laser beams are incident on the polygon mirror 19 as shown by the arrow (D) and reflected thereby. During rotation of the sleeve 13, sleeve 13 rotates at the high speed with a gas pressure increasing between the herringbone-configured hydrodynamic pressure generating groove 12A and the sleeve 13 due to a pumping action of the herringbone-configurated hydrodynamic pressure generating grooves 12A. A part of the gas thus pressurized is introduced from the peripheral groove 12B to the free end of the shaft 12 via the communicating hole 12. Then, blown out toward the lid 17 through the throttling hole 12D consisting of a slit. Thus, a force for floating the sleeve 13 relative the shaft 12 in the direction shown by the arrow (A) is generated. The weight of the sleeve 13, the mirror 19, and the lid 17 and the force of the magnet 15 for attracting the stator 14 act in the direction shown by the arrow (B). As a result, the forces acting in the direction shown by the arrows (A) and (B) balance each other. Thus, the sleeve 13 floats in an amount shown by the arrow (C). FIG. 8 shows the distribution of the gas pressure in the radial direction generated by the herringbone-configured hydrodynamic pressure generating grooves 12A and that of the pressure in the thrust direction generated by the blow-out of the gas from the communicating hole 12C. The protective cover 20 and the glass 20A prevent dust from penetrating into the bearing and dew from being formed on the polygon mirror 19.
However, the above construction has the following disadvantages: The gas pressure in the vicinity of the fixed end of the shaft 12 does not increase sufficiently and thus the fixed end of the shaft 12 seizes and a whirl or a self-excited vibration is generated, which may lead to an unstable rotation of the bearing. Particularly, the communication of the herringbone-configured hydrodynamic pressure generating grooves 12A with the peripheral groove 12B or the communicating hole 12C reduces pressure of the gas and generates the whirl to a great extent.